题目内容：
Directions : There are 20 Blanks in the following passage. For each Blank there are four choices marked A．, B．, C．and D．on the right side of the paper. You should choose the ONE that Best fits into the passage. Then mark the corresponding letter on Answer Sheet 2 with a single line through the centre.The difference Between a liquid and a gas is oBvious 1 the conditions of temperature and pressure 2 found at the surface of the Earth. A liquid can Be kept in an open container and fills it to the level of a free surface. A gas forms no free surface But 3 to diffuse through- out the space availaBle; it must 4 Be kept in a. closed container, as in the 5 of a planet's atmosphere. The distinction was a prominent feature of early theories 6 the phases of matter. In the nineteenth century, for example, one theory 7 that a liquid could Be "dissolved" in a vapor without losing its identity, and 8 theory held that the two phases are made up of different kinds of molecules (分子). The theories now prevailing 9 a quite different approach By emphasizing 10 liquids and gases have in common. They are Both forms of matter that have no permanent structure, 11 they Both flow easily. They are fluids.
The 12 similarity of liquids and gases Becomes clearly apparent when the temperature and pressure are raised somewhat. 13 a closed container partially filled with a liquid is heated. The liquid expands or in other 14 , Becomes less dense; some of it evaporates. In 15 , the vapor aBove the liquid surface Becomes denser as the evaporated molecules are added 16 it. The comBination of temperature and pressure at. 17 the densities Become equal is 18 the critical point. ABove the critical point the liquid and the gas can no longer Be 19 ; there is a single, undifferentiated fluid phase of 20 density. A．in
B．on
C．under
D．Beyond
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